Past Mega-Quakes Left Mark on Canadian Coast

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Thanks to decades of geologic detective work, scientists know
that on Jan. 26, 1700, at 9 p.m., a massive earthquake and
tsunami hit the Pacific Northwest.

Born from the
Cascadia Subduction Zone, the earthquake may have cleaved the
620-mile-long (1,000 kilometers) offshore fault from Northern
California to Canada. Researchers don't yet know; they must play
connect the dots with clues left behind in billowy layers of sand
and mud.

A group of Canadian geologists is connecting some of these dots,
with the first record of past earthquakes
from the Pacific Coast of Vancouver Island. The team discovered
evidence of 21 temblors in the past 11,000 years, including the
1700 quake and a 1946 shaker centered on the island. The new
findings are detailed in the June 12 issue of the Canadian
Journal of Earth Sciences.

Earthquake archive

The history of Canadian earthquakes adds to an archive being
assembled of the Cascadia Subduction Zone, which marks the
collision between the North American and Juan de Fuca tectonic
plates. Subduction zones, where one plate slides beneath another,
trigger the biggest earthquakes and tsunamis on the planet, such
as the 2004 Sumatra and 2011 Japan disasters. The 1700 megathrust
quake likely rivaled those two catastrophic quakes in size,
researchers think. [ The
10 Biggest Earthquakes in History ]

The Vancouver Island records match up with16
past earthquakes discovered in disturbed seafloor sediments
offshore of southern Vancouver Island, Washington and Oregon, the
researchers said. But not all of the ancient quakes seen along
the southern part of the subduction zone had a counterpart in the
new record.

"Perhaps not every megathrust earthquake is equal to another,"
said Audrey Dallimore, study co-author and a marine geologist at
Royal Roads University in Victoria, B.C. "Some may only rupture
the southern part of the zone."

The new earthquake record is from Effingham Inlet, a former
glacial fjord in Barclay Sound, on the southwestern coast of
Vancouver Island. A crew shoehorned an ocean drilling ship into
the narrow inlet and drilled down to bedrock, pulling up 138 feet
(42 meters) of sediment in what is called a core. Because there's
little oxygen in the inlet's bottom waters, no marine creatures
mix up the sediments, leaving a nearly pristine archive of the
past.

"The sediments are laid down year by year like tree rings,"
Dallimore said. "These millimeter-thick layers go back thousands
and thousands of years."

Two clusters of quakes

The disturbed layers left by earthquakes aren't evenly
distributed through time, but rather repeat about every 200 years
and every 900 years (called
recurrence intervals ). Carbon dating of organic material,
along with ash from Mount Mazama's eruption in 5,677 B.C.,
precisely dates the layers. (Mount Mazama is part of the Cascade
Volcanic arc in Oregon.)

The bimodal pattern (of 200 and 900 years) could reflect the
island's two earthquake sources — the offshore subduction zone
and local faults, such as the one that caused the 1946
earthquake, Dallimore said. But evidence elsewhere on the
Cascadia Subduction Zone also suggests the giant fault rips apart
at irregular intervals.

"We know the longest time between earthquakes [in Effingham
Inlet] is 1,000 years. The next earthquake could be tomorrow or
it could be 700 years from now," Dallimore said.

Filling in gaps

To complete the picture, Dallimore and her colleagues have
collected more samples from inlets further north on Vancouver
Island. The researchers plan to compare those cores to Effingham
Inlet and American sites to better understand how the Cascadia
Subduction Zone ruptures along the coast.

Chris Goldfinger, a geologist at Oregon State University who was
not involved in the research, said the study "was a very nice
piece of work."

"It's becoming more and more clear that big [Cascadia]
subduction earthquakes are reliably recorded in many
environments, something that will eventually allow us to combine
all these data and estimate slip models and magnitudes for past
earthquakes," Goldfinger said.